Ch t 9Chapter 9 - Seoul National University

Ch t 9Chapter 9Unit Operations

and Pollution Prevention

Contents

9 1 I d i9.1 Introduction

9.2 Pollution Prevention in Material Selection for Unit Operations

9.3 Pollution Prevention for Chemical Reactors

9.4 Pollution Prevention for Separation Devices

9.5 Pollution Prevention Applications for Separative Reactorspp p

9.6 Pollution Prevention in Storage Tanks and Fugitive Sources

9.7 Assessment Integrated with HAZ-OP

9.8 Integrated Risk Assessment with Design9.8 teg ated s ssess e t w t es g

9.1 Introduction

In developing a flowsheet for the production of a chemical, it isp g p ,desirable to consider the environmental ramification of eachunit operation in the process rather than postponing this

id ti til th fl h t i fi i h d Thi “f t d”consideration until the flowsheet is finished. This “front end”environmental assessment is more likely to result in achemical process that has less potential to causechemical process that has less potential to causeenvironmental harm.

In many instances, this environmentally benign design willalso be more profitable, because the improved design will

i l t t t t d i t l lirequire lower waste treatment and environmental compliancecosts and will convert a higher percentage of raw materialsinto salable productinto salable product.ramification: 결과

9.1 Introduction

In considering pollution prevention for unit operations in the design of

chemical processes, the following considerations are important.

F t E d P ll ti P ti C id ti Front-End Pollution Prevention Considerations.

1 M t i l S l ti1. Material Selections

2. Waste Generation Mechanisms

3. Operation Conditions

4 Material Storage and Transfer4. Material Storage and Transfer

5. Energy consumption

6. Process Safety

9.1 Introduction

1. Material Selections: Many of the environmental concerns can beaddressed by reviewing materials properties and making correct choiceof unit operation & conditions. The materials used in each unitoperation should be carefully considered so as to minimize the humanoperation should be carefully considered so as to minimize the humanimpact and environmental damages of any releases that might occur.

2. Waste Generation Mechanisms: Often, a careful evaluation of themechanisms of in-process waste generation can direct the processd i d i ll d i l h i d hdesigner toward environmentally sound material choices and otherpollution prevention options.

3. Operation Conditions: The operating conditions of each unit should beoptimized in order to achieve maximum reactor conversion andseparation efficiencies

9.1 Introduction

4. Material Storage and Transfer: The best material storage andt f t h l i h ld b id d i d t i i itransfer technologies should be considered in order to minimizereleases of materials to environmental.

5. Energy consumption: Energy consumption in each unit should becarefully reviewed so as to reasonably minimize its use and theassociated release of utility related emissionsassociated release of utility-related emissions.

6. Process Safety: The safety ramifications of pollution preventionmeasures need to be reviewed in order to maintain safe workingconditions.

In following sections, we apply this framework for preventing pollutionin unit operations by considering choices in materials, technologyselection, energy consumption, and safety ramifications.

9.1 Introduction

“RISK SHIFTING 1”

Pollution prevention decisions that are targeted to reduce one kindof risk may increase the level of risk in other areas. For example, ay pcommon method of conserving water resources at chemicalmanufacturing facilities is employ cooling towers. Process waterused for cooling purposes can be recycled and reused many times.used for cooling purposes can be recycled and reused many times.

However, there is an increased risk for workers who may exposedto the biocide used to control microbial growth in the cooling waterto the biocide used to control microbial growth in the cooling watercircuit (anti-fungal biocide, e.g., copper-8-quinolinolate).

Also in some cooling water processes hazardous waste is createdAlso, in some cooling water processes, hazardous waste is createdby the accumulation of solids: for example, from the use ofhexavalent chromium (a cancer causing agent) as a corrosioni hibitinhibitor.

Cooling tower

cooling towers

9.1 Introduction

“RISK SHIFTING 2”

Another example of shifting risk from the environment and thegeneral population to workers involves fugitive sourcesgeneral population to workers involves fugitive sources(valves, pumps, pipe connectors, etc.).

O t t f d i f iti i i i d thOne strategy for decreasing fugitive emissions is reduce thenumber of these units by eliminating backup units andredundancy. This strategy will decrease routine air releasesy gybut will increase the probability of catastrophic release orother safety incidents.

Simply put, the objective of pollution prevention is to reducethe overall level of risk in all areas and not to shift from onetype to another.


One important element of designing and modifying process units for

p

p g g y g ppollution prevention is the choice of materials that are used inchemical processes. These materials are used as feedstocks,solvents, reactants, mass separation agents, diluents, and fuels.

In considering their suitability as process components it is notIn considering their suitability as process components, it is notsufficient to consider only material properties that are directly relatedto processing; it is increasingly important to consider thep g; g y penvironmental and safety properties as well.

Use of materials that are known to be persistent, bioaccumulative,or toxic should be avoided as they are under increasing regulatoryscrutiny and many manufactures are moving away from their usescrutiny and many manufactures are moving away from their use.


Questions regarding material selection include:

p

g g

a) What are the environmental, toxicological, and safetyproperties of the material ?properties of the material ?

b) How do these properties compare to alternative choices ?c) To what extent does the material contribute to waste

generation or emission release in the process ?d) Are there alternative choices that generate less waste or emit

less while maintaining or enhancing the overall yield of theless while maintaining or enhancing the overall yield of thedesired product ?

If processing materials can be found which generate less wasteand if the hazardous characteristics of those wastes are lessproblematic then significant progress may be made in preventingproblematic, then significant progress may be made in preventingpollution from the chemical process.


Materials are involved in a wide range of processing functions in

p

Materials are involved in a wide range of processing functions inchemical manufacturing, and depending on the specific application,their environmental impacts vary greatlytheir environmental impacts vary greatly.

For example reactants for producing a particular chemical canFor example, reactants for producing a particular chemical canvary significantly with respect to toxicity and inherent environmentalimpact potential (global warming ozone depletion etc ) and theyimpact potential (global warming, ozone depletion, etc.,), and theycan exhibit various degrees of selectivity and yield toward thedesired product In addition the properties of reaction byproductsdesired product. In addition, the properties of reaction byproductscan vary widely, similarly to reactants.


Some catalysts are composed of hazardous materials or they may

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react to form hazardous substances. For example, catalysts used forhydrogenation of carbon monoxide can form volatile metal carbonyl

d h i k l b l th t hi hl t icompounds, such as nickel carbonyl, that are highly toxic.

Many catalysts contain heavy metals and environmentally safeMany catalysts contain heavy metals, and environmentally safedisposal has become an increasing concern and expense. After thedeposition of inhibitory substances the regeneration of certaindeposition of inhibitory substances, the regeneration of certainheterogeneous catalysts releases significantly amounts of SOx, NOx,and PM. For example, the regenerator for a fluid catalytic crackingand PM. For example, the regenerator for a fluid catalytic crackingunit (FCCU) is a major source of air pollution at refineries. Note thatthe removal of the FCCU would result in very low yields andy yconsequently unacceptable waste generation at facility level.


The choice of a mass separating agent for solids leaching or

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p g g gliquid extraction applications can affect the environmental impactsof those unit operations. Agents that are matched well to thedesired separation will consume less energy and release lessdesired separation will consume less energy and release lessenergy-related pollutants than those that are not well suited for theapplication. Typically, agents that have lower toxicity will requireless stringent clean up levels for any waste streams that aregenerated in the process.

The choice of fuel for combustion in industrial boilers willdetermine the degree of air pollution abatement needed to meetenvironmental regulations for those waste streams. As anillustration, using fuel types having lower sulfur, nitrogen, and tracemetals levels will yield a flue gas with lower concentrations of acidmetals levels will yield a flue gas with lower concentrations of acidrain precursors (SOx and NOx) and particulate matter.


Example 9.2-1 : Compare the emission of SO2 resulting from the combustion of three fueltypes that will satisfy an energy demand of 106 BTU.

p

No. 6 F.O. No. 2 F.O. Natural GasDensity (lb/ft3) 61.23 53.66 0.0485Heating Value (BTU/gal) 148,000 130,000 1060 BTU/ft3g ( g ) , ,Carbon (wt%) 87.27 87.30 74.8Hydrogen (wt%) 10.49 12.60 25.23Sulfur (wt%) 0.84 0.22 0Oxygen (wt%) 0 64 0 04 0 0073Oxygen (wt%) 0.64 0.04 0.0073Nitrogen (wt%) 0.28 0.006Ash (wt%) 0.04 <0.01

No. 6 F.O.106 BTU/148,000 BTU/gal=6.76 gal = 55.34 lbSO2 generated = (55.34 lb)(0.0084 lb S/lb)(64.06 lb SO2/32.06 lb S)=0.929 lb SO2

No. 2 F.O.106 BTU/130,000 BTU/gal=7.69 gal = 62.95 lbSO2 generated = (62.95 lb)(0.0022 lb S/lb)(64.06 lb SO2/32.06 lb S)=0.276 lb SO2SO2 generated (62.95 lb)(0.0022 lb S/lb)(64.06 lb SO2/32.06 lb S) 0.276 lb SO2

Natural GasSO2 generated = 0.0 lb


Less toxic materials, such as water and air, can still have important

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environmental implications due to the waste streams that aregenerated in their use.

Air is often used in chemical reactions either as a diluent or as asource of oxygen. For certain high temperature reactions, the N2and O2 molecules in the air react forming oxides of nitrogen Uponand O2 molecules in the air react, forming oxides of nitrogen. Uponrelease, NOx will participate in photochemical smog reactions in thelower atmosphere. Therefore, it is important to consider alternativesources of oxidants such as enriched air or pure oxygen andsources of oxidants, such as enriched air or pure oxygen, anddiluents, such as CO2 or other inert reaction byproducts.

Water is used for many purposes in chemical processes; as boilerWater is used for many purposes in chemical processes; as boilerfeed, a cooling medium, reactant, or a mass separating agent. Thefollowing example illustrates that the quality of the feed water canh f d i fl th ti f h d t ihave a profound influence on the generation of hazardous waste ina refinery

9.2 Pollution Prevention in Material Selection for Unit Operationsp

Example 9.2-2Solid accumulate in the boiler and excessive levels of suspended solidsSolid accumulate in the boiler and excessive levels of suspended solidslead to fouling of heat transfer surfaces in the process, a decrease in heattransfer efficiency, and requires periodic shut-down and cleaning of thesesurfaces to restore normal operation.

To control solids accumulation the content of the boiler are sent toTo control solids accumulation, the content of the boiler are sent towastewater treatment when the dissolved solids content is above a cut-offlevel, in a step termed “blow-down”

Use all process water using RO to separate dissolved solids !

9.2 Pollution Prevention in Material Selection for Unit Operationsp

9.3 Pollution Prevention for Chemical reactors

From an environmental perspective, reactors are the most i t t it ti i h i limportant unit operation in a chemical process

Degree of conversion of feed to desired products influencesg p~ all subsequent separation processes, ~ recycle structure for reactors, ~ waste treatment options~ waste treatment options, ~ energy consumption, and~ ultimate pollutant releases to the environment.

Reactor considerations with pollution prevention idea:1) material use and selection1) material use and selection2) reaction type and reactor choice, and3) reactor operation

(Table 9.2-1)

Material Use and Selection for Reactors

Issues involving the use of materials in the light of

their influence on environmental impact of reactors

raw materials and feedstocks

t l t catalyst

solvent, diluents, etc., ,

Table 9.2-1Summary of material selection issues for unit operations in chemical processeschemical processes

Raw Materials and Feedstocks

Raw materials can be highly toxic or cause undesirable Raw materials can be highly toxic or cause undesirablebyproducts and their presence be a concern of theuncontrolled release and exposure to humans (eliminate asmany of these toxic raw materials intermediates and productsmany of these toxic raw materials, intermediates and productsas possible)

May necessitate the adoption of new process chemistry(Phosgene free process for polycarbonates)

In the production of fuels for transportation, petroleumrefineries are required to remove sulfur from products (if not,SO i l i d d )SO2 is releases to air and damage ecosystems)

In partial oxidation of hydrocarbons and air is the source ofp yO2 that produce NOx (precursors of smog)

Solvents

M t l ti d l i l i ti i Most solution and emulsion polymerization requires solvents (enhance reactions, co-solubilize monomers,

diluting medium to modulate reactions)

Substitution of environmentally-benign solvents (i t f l bilit t i l i l t i t l(in terms of solubility, toxicological, cost, environmental properties)

Supercritical Fluids

Catalysts

Catalysts are either homogeneous or heterogeneous y g g(choice has large impact on the efficiency of the reactor and environment)

Catalysts can allow the use of more environmentally-benign chemicals as raw materialsbenign chemicals as raw materials

Catalysts can increase selectivity toward the desired Catalysts can increase selectivity toward the desired product and away from unwanted byproducts (waste)

They can convert waste chemicals to raw materials and create more environmentally acceptable products di l f idirectly from reactions

Catalysts

Example:

Production of reformulated gasoline (RFG) and diesel fuels from crude oil

How improved catalyst can create chemicals that are better for environment

Trends:Trends:

a) increased processing of crude oils with lower qualitya) increased processing of crude oils with lower quality(higher % of sulfur, N, metals and carbon residues )

b) more demand of lighter fuels and less for heavy oils) g yc) environmental regulations that limit % of sulfur, heavy

metals, aromatics, VOC in transportation fuels

(Table 9.3-1)

Table 9.3-1

Summary of conventional and improvedand improved catalysts for reformulated gasoline andgasoline and diesel production

Reaction Type and Reactor Choice pp261

Reaction mechanism (order, series of parallel reaction pathways, reversible or not ) influences pollution prevention and strategies

Mechanism details determine optimum temp, RTD and mixing Reactor operation influences conversion, selectivity, yield, by-

product and waste formation For pollution prevention very high yield high selectivity for For pollution prevention, very high yield, high selectivity for

desired product and low selectivity for byproducts.

Typical reactor efficiency measure for reaction yield (ratio of exiting product concentration to inlet reactant, ( [P] / [R]0 );

frange from 0~1 Modified selectivity as the ratio of exiting product concentration

to the sum of product and byproduct (waste)to the sum of product and byproduct (waste), ([P] / ([P]+[W]) = [P] / [ Reactant consumed ]; range from 0~1

Parallel ReactionsParallel ReactionsParallel Reaction

D (Desired Product)A

kDThe economic incentive

AU (Undesired Product)kU

Maximize the formation of DMinimize the formation of U

Competing or side rxnor side rxn

D

A

DTotal cost

Reaction costA

reaction

separation

Separation cost

A, ULow High

Rxn-separation system producing both D & U Efficiency of a reactor system

Rate selectivity parameter, SRate selectivity parameter, SRate selectivity parameter, S

D (Desired Product)A

kD1

ADD Ckr A

U (Undesired Product)kU2

AUU Ckr

The rate laws are

21 AUADA CkCkrSelectivity tells us how one product is

2

1

ADD

AUADA

Ckr

Ckr

CkCkr how one product is favored over another

when we have multiple reactions.

2 AUU

kr

Ckr

Rate of formation of D21 A

U

D

U

DDU C

kk

rrS = Rate of formation of U

Rate of formation of D

Rate selectivity parameter = Instantaneous selectivity

Maximizing SDU for one reactant

Case 1: Case 1: 1 1 > > 2 2 , a = , a = 11-- 2 2

21A

U

D

U

DDU C

kk

rrS

aA

DDDU C

krS A

UUDU C

krS

maximize Smaximize SDUDU

- keeping the concentration of reactant A as high as possible during the rxn

- in gas phase rxn, we should run it without inerts and at high pressures to keep CA high

- in liquid phase rxn, the use of diluents should be keep to a minimum

- use a batch or plug-flow reactorp g

Maximizing SDU for one reactant

C 2C 2 >>

21A

U

D

U

DDU C

kk

rrS

Case 2: Case 2: 2 2 > > 1 1 , a = , a = 22-- 1 1

k 1aAU

D

U

DDU Ck

krrS 1

maximize Smaximize SDUDUmaximize Smaximize SDUDU

- keeping the concentration of reactant A as low as possible during the rxn

- in gas phase rxn, we should run it with inerts and at low pressures to keep CA low

- in liquid phase rxn, the use of diluents should be keep to a maximum

- use a CSTR or recycle reactor (product stream act as a diluent)

Maximizing SDU for one reactantWhether the reaction should be run at high or low T

EE

Whether the reaction should be run at high or low T

21A

DDDU C

kkrS

RTEE

U

D

U

DUD

eAA

kk

UU kr

Case 3: ECase 3: ED D > E> EU U → → high Thigh TUDS /

UD EE

- kD (rD) increases more rapidly increasing temperature than does the kU.

- keeping the temperature as high as possible to maximize SDU.

UDS /

T

Case 4: ECase 4: EDD < E< EUU→ low T→ low T

T

UD EE Case 4: ECase 4: ED D E EU U low T low T

- keeping the temperature as low as possible to maximize SDU

- not so low that the desired rxn does not proceed to any significant extent

UDS /

not so low that the desired rxn does not proceed to any significant extent.

T

Ex 6Ex 6--1 Minimizing unwanted products (U & Q) for a single reactant1 Minimizing unwanted products (U & Q) for a single reactant

DD (1)(1)T C

13001000,36

00020DD (1)(1)

AA UU (2)(2)

AT

D Cer 3000002.0

5.11

3001000,25

0018.0 AT

U Cer

QQ (3)(3)5.01

3001000,5

00452.0 AT

Q Cer

- Pre-exponential factors are comparable- the activation energies of (1) & (2) are much greater than that of (3).- at high temperature rQ << rD, rUg p Q D, U

(4)(4)largeveryQ

DDQ r

rS DYXD rr

rS

)/( ( )( )

(5)(5)5.0

13001000,11

11.0

A

T

U

DDU

Q

Ce

rrS

YX rr

- From (5), SDU minimized at low concentration of A

AU

1. High Temperatures (to minimize the formation of Q)1. High Temperatures (to minimize the formation of Q)1. High Temperatures (to minimize the formation of Q)1. High Temperatures (to minimize the formation of Q)2. Low concentration of A (to minimize the formation of U)2. Low concentration of A (to minimize the formation of U)

-- Adding inerts, using low pressures, using CSTR or recycled reactorAdding inerts, using low pressures, using CSTR or recycled reactor

Rate selectivity parameter, SRate selectivity parameter, SMaximizing SDU for two reactants

D (Desired Product)A B

k1

A + BU (Undesired Product)k2

The rate laws are

221121

BABAA CCkCCkr

22

111

21

BAD

BABAA

CCkr

CCkr

CCkCCkr

222

BAU

k

CCkr

2121

2

1 BAU

DDU CC

kk

rrS

Rate selectivity parameter=Instantaneous selectivity

Different reactors and schemes for minimizing the unwanted productDifferent reactors and schemes for minimizing the unwanted productFigure 6Figure 6--33

AB

A

B A

B AB A B

(a) CSTR (b) tubular reactor (c ) batch (d) semibatch 1 (e) semibatch 2

AB

ABB B

(f) Tubular reactor with side streams (g) Tubular reactor with side streams

AB

BA

(h) Tubular reactor with recycle (i) Series of small CSTRs

Ex. 6Ex. 6--2 Minimizing unwanted products for two reactants2 Minimizing unwanted products for two reactants

for the parallel reactionD (Desired Product)k1 ( )


Case I : 1 > 2, 1 > 2, a = 1-2 > 0, b = 1-2 > 0

th t l ti it tbB

aA

DDU CCkrS 1the rate selectivity parameter BA

UDU CC

krS

2

To maximize the S maintain the concentration of both A and B as high as possibleTo maximize the SDU, maintain the concentration of both A and B as high as possible

a tubular reactora tubular reactor

a batch reactora batch reactor a batch reactora batch reactor

high pressures ( if gas phase )high pressures ( if gas phase )




Case II : 1 > 2, 1 < 2, a = 1-2 > 0, b = 2-1 > 0

th t l ti it t

aAD CkrS 1the rate selectivity parameter bBU

DU CkrS

2

To maximize the S maintain C high and C lowTo maximize the SDU, maintain CA high and CB low.

a semibatch reactor in which B is fed slowly into A. (Fig. 6a semibatch reactor in which B is fed slowly into A. (Fig. 6--3d)3d)

a tubular reactor with side stream of B continually (Fig 6a tubular reactor with side stream of B continually (Fig 6--3f)3f) a tubular reactor with side stream of B continually (Fig. 6a tubular reactor with side stream of B continually (Fig. 6 3f)3f)

a series of small CSTRs with A fed only to the first reactor (Fig. 6a series of small CSTRs with A fed only to the first reactor (Fig. 6--3i)3i)


for the parallel reactionD (Desired Product)

A + Bk1


Case III : 1 < 2, 1 < 2, a = 2-1 > 0, b = 2-1 > 0

the rate selectivity parameterbB

aAU

DDU CCk

krrS

2

1

To maximize the SDU, maintain the concentration of both A and B as low as possible

a CSTRa CSTR a CSTRa CSTR

a tubular reactor in which there is a large recycle ratioa tubular reactor in which there is a large recycle ratio

a feed diluted with inertsa feed diluted with inerts a feed diluted with inertsa feed diluted with inerts

low pressures ( if gas phase )low pressures ( if gas phase )




Case IV : 1 < 2, 1 > 2, a = 2-1 > 0, b = 1-2 > 0

th t l ti it t

bBD CkrS 1the rate selectivity parameter aAU

DU CkrS

2

To maximize the S maintain the concentration of both A and B as high as possibleTo maximize the SDU, maintain the concentration of both A and B as high as possible

a semibatch reactor in which A is slowly fed to B (Fig. 6a semibatch reactor in which A is slowly fed to B (Fig. 6--3e)3e)

a tubular reactor with side stream of A (Fig 6a tubular reactor with side stream of A (Fig 6--3g)3g) a tubular reactor with side stream of A (Fig. 6a tubular reactor with side stream of A (Fig. 6 3g)3g)

a series of small CSTRs with B fed only to the first reactor (reverse of Fig. 6a series of small CSTRs with B fed only to the first reactor (reverse of Fig. 6--3i)3i)

Maximizing the desired product in series reaction

k1 k2A B C

In parallel rxns maximize the desired product

A B C

In parallel rxns, maximize the desired product

adjusting the reaction conditions

choosing the proper reactor

In series rxns, maximize the desired product

adjusting the space-time for a flow reactor

choosing real-time for a batch reactor g

Maximizing the desired product in series reaction

k1 k2A B C

If th fi t ti i l d d ti i f t it

A B C

If the first reaction is slow and second reaction is fast, itwill be extremely difficult to produce species B.

If the first reaction (formation of B) is fast and the reactionto form C is slo a large ield of B can be achie edto form C is slow, a large yield of B can be achieved.

However, if the reaction is allowed to proceed for a longtime in a batch reactor or if the tubular flow reactor is toolong the desired product B will be converted to Clong, the desired product B will be converted to C.

Ex. 6Ex. 6--3 Maximizing the yield of the intermediate product3 Maximizing the yield of the intermediate product

The oxidation of ethanol to form acetaldehyde is carriedout on a catalyst of 4 wt% Cu-2wt% Cr on Al2O3.y 2 3Unfortunately, acetaldehyde is also oxidized on thiscatalyst to form carbon dioxide. The reaction is carriedout in a dilute concentrations (ca. 0.1% ethanol, 1% O2,and 98.9% N2). Consequently, the volume change withthe reaction can be neglected Determine thethe reaction can be neglected. Determine theconcentration of acetaldehyde as a function of spacetime.

k1 k2CH3CH2OH(g) CH3CHO 2CO2OHO 222

1 OHO 22 2

25

2 2

The rxns are irreversible and first-order in ethanol andacetaldehyde respectivelyacetaldehyde, respectively.

Solution

τk1CC τk0AA

1eCC

12

τkτk

0A1B kkeeCkC

21

12 kk

Reaction paths for different ks in series reaction Reaction paths for different ks in series reaction

A B Ck1 k2

11 kBFor k1/k2>1, a

Large quantity of B 12

1 k

Large quantity of BCan be obtained

1~2

1

kk

For k1/k2<1, a

12

1 kk

'1

'2

For k1/k2 1, aLittle quantity of BCan be obtained 1st rxn is slow

2nd rxn is fast

'3

2k

A C

1

A CLong rxn time in batch or long tubular reactor-> B will be converted to C

Examples :

1. Parallel reaction pathway

(partial oxidation of ethylene to ethylene oxide, whereas parallel(partial oxidation of ethylene to ethylene oxide, whereas parallel

reaction converts ethylene to by-products, CO2 and water)

Pollution generation with irreversible 1st order parallel reaction

R P

R W

kp

kwR Ww

where R is reactant, P is product and W, the waste byproductkp and kw are the 1st order reaction rate constantp w

Parallel reaction pathway

R P

R W

kp

kw

Relative concentrations of P and W are affected by kp / kw (Fig. 9.3-1)

R Ww

y p w ( g )

To achieve max yields, RTD must be about 5 times the reaction ytime constants (kp + kw ) -1

Reaction selectivity is constant and independent of RTD of 1st

order irreversible isothermal parallel reactionsorder irreversible isothermal parallel reactions.

In reality selectivity is dependent on the RTD and this parameter is In reality, selectivity is dependent on the RTD and this parameter is

the factor for pollution prevention

R P

R W

kp

kwR W

Fig 9 3 1Fig. 9.3-1Effect of ratio of kp/kw on the reactor outlet concentrations of products, reactants, andbyproducts for a simple irreversible 1st order parallel reaction mechanism

Series reaction pathway

R P Wkp kw

Rate of waste generation depends on the rate of production formation

(Fig. 9.3-2)

Longer RTD leads to not only more product formation but also

more byproduct generation

Amo nt of aste generation depends on the ratio of k / k and RTD Amount of waste generation depends on the ratio of kp / kw and RTD

For each ratio, kp / kw there is an optimum RTD that maximize

the product concentration

R P Wkp kwR P W

Fig. 9.3-2Eff t f d t d t ti t t t d t d t t ti i 1 tEffect of product and waste reaction rate constants on product and waste concentrations in a 1st

irreversible series reaction. The RTD has been made dimensionless using the product reactionrate constant

2. Series reaction pathway2. Series reaction pathwaySeries reaction pathway

Product yield and modified selectivity continues toy ydecrease with time (Fig. 9.3-3) in series reaction

At longer RTD, the rate of waste generation is greater than therate of production formation

To minimize waste generation in series reaction, operate reactorso that the ratio kp/kw is as large as possible and to control the RTD

By removing product as it is being formed and before itsconcentration builds up in the reactor, then the byproduct can be

i i i dminimized

R P Wkp kwR P Ww

Fig 9 3-3Fig. 9.3-3Effect of product and waste reaction rate constants on product yield and modified selectivity for 1st order irreversible series reaction.

2. Series reaction pathway2. Series reaction pathwayReversible Reaction pathway

Another important category of reactions (Fig 9 3-4) in parallel Another important category of reactions (Fig. 9.3-4) in parallel and series modes

Reversible reactions inhibit full conversion of reactants

RTD i k ti t RTD is a key operating parameter

Selectivity improvement for reversible reaction, operated at y p , pequilibrium, can be achieved by utilizing the concept of recycle to extinction ( Example ) steam reforming of methanerecycle to extinction.( Example ) steam reforming of methane

Fig. 9.3-4Product and byproduct profiles for reversible parallel and series reactions

Reversible Reaction pathway

224 3HCOOHCH

222 HCOOHCO

Both reactions are reversible at equilibrium

When CO2 is recovered and recycled back to the reactor, itdecomposes in the reactor as fast as it forms

No net conversion of CH4 to CO2 occurs

This requires additional operating costs, but there is noselectivity loss of reactant, the process is cleaner

It may be lowest cost option overall

Reactor combined with a separator that recycles reactants andReactor combined with a separator that recycles reactants and byproducts (Fig. 9.3-5)

Can be operated such that all reactants fed to the reactor are Can be operated such that all reactants fed to the reactor are converted to product with no net waste generation

S l ti it i t b b ti t Selectivity improvements can be by separative reactors

P

Reactor SeparatorRR, P, W

P

p

R, W

Fig. 9.3-5Process flow diagram illustrating the concept of recycling to extinction for reversible reactions

Reactor Operation

Reaction Temperature

Reaction Temp. influence the conversion, yield and selectivity

For irreversible 1st order parallel reaction the selectivity indicator is For irreversible 1st order parallel reaction, the selectivity indicator is

(E

)/(

)/(

][][

RTwEw

RTpEp

w

p

w

p

eAeA

kk

RkRk

where A the frequency factor and

E the activation energy

1/)()/()/( RTEERTERTE

0/)(

1/)(

)0/()0/(

)1/()1/(

//

RTwEpE

RTwEpE

RTwERTpE

RTwERTpE

w

p

ee

eeee

kk

Reactor Operation

Reaction Temperature

Calculate change of the kp/kw ratio as the function of T change g p w gto a new T1 w.r.t. given T0 (Fig. 9.3-6)

when E > E the ratio increases with with increasing T and decreases when Ep > Ew the ratio increases with with increasing T and decreases with decreasing T

Thus pollution can be prevented in parallel rxn by increasing reactor T Thus, pollution can be prevented in parallel rxn by increasing reactor T when Ep > Ew

Th it h ld t h E < E The opposite holds true when Ep < Ew

when the difference Ep and Ew increases, T has more influence

Fig. 9.3-6gEffect of T on the ratio of rate constants for 1st order parallel and irreversible reaction

Reactor Operation

Mixing( Complex multiple reactions can be influenced by the intensity of mixing in CSTR )

F i ibl ti th i ld d l ti it b For irreversible reaction, the yield and selectivity may be altered compared to the case where the reactants are mixed i t t l t l l l l ( b t tinstantaneously to a molecular level (may be generate waste byproduct a lot)

Rate can be reduced due to diffusional limitations between segregated elements of the reacting mixture

Imperfect mixing is particularly evident for rapidly reacting systems

Competitive consecutive reaction in CSTR (series parallel reaction)

k1A + B Rk

# kinetics of nitration and halogenation of hydrocarbons saponification of polyesters

R + B Sk2

A is initially charged and B is added continuously as a solution

hydrocarbons, saponification of polyesters

y g y

R is the desired product and S is byproduct

If rxn is 1st order mixing will not affect selectivity If rxn is 1st order, mixing will not affect selectivity

If it is 2nd order, the presence of local excess C can cause

overreaction of R to S This mixing effect occurs for both homo-

and heterogeneous reaction systems for batch or semi-batch

reactor

Ex. Iodization of L-tyrosine in aqueous solution (Fig. 9.3-7)d f ff f T i i i l f A (A ) f ddi i f B study of effects of T, initial conc. of A (A0), rate of addition of B,

agitation rate of the vessel impeller, presence or absence of baffling in the reactorbaffling in the reactor.

correlation for all of these parameters was found between the ratio of measured yield to expected yield(Y / Yexp) vs. dimensionless f y p y ( exp)quantity, ( k1B0 ) ( A0 / B0) ,

where k1 : product reaction rate constant (L/gmol sec)where k1 : product reaction rate constant (L/gmol.sec)k2 : by product reaction rate constant (L/gmol.sec) : microtime scale for mixing of eddies of pure B with bulk

li id( )liquid(sec)Y : measure yield (R/A0 )

(k1B0 ) : extent of conversion of A and B under conditions of i l ipartial segregation

A/A0 : fraction of reactant A remaining at the end of the reaction

Y d l d ( f ) R/AYexp : expected yiled (perfect mixing) = R/A0

C l ti fitti th d t i iC l ti fitti th d t i i Fi 9 3Fi 9 3 88 Correlation fitting the date is in Correlation fitting the date is in Fig. 9.3Fig. 9.3--88-- measured Y/Ymeasured Y/Yexpexp = 0.66~0.98 depends on mixing intensity and = 0.66~0.98 depends on mixing intensity and

hhother parametersother parameters-- when, ( kwhen, ( k11BB0 0 ) ( A) ( A0 0 / B/ B00) ) 10 10 --55 , then Y , then Y YYexpexp , this criterion , this criterion

allow usallow usallow usallow us-- mixing intensity for 2mixing intensity for 2ndnd order competitiveorder competitive--consecutive rxnconsecutive rxn

1010 ––5 5 == ( k( k BB ) ( A) ( A / B/ B ) = ( k) = ( k AA ))10 10 ( k( k11BB0 0 ) ( A) ( A0 0 / B/ B0 0 ) ( k) ( k1 1 AA0 0 ) ) -- rearranging and incorporating the Kolmogoroff theory rearranging and incorporating the Kolmogoroff theory

4/7

4/34/35

)'(882.010

uL

kAf

10 )'(ukA

Fig. 9.3-7 Iodization of L-tyrosine in aqueous solution

correlation fitting the date is in correlation fitting the date is in Fig. 9.3Fig. 9.3--8 8 rearranging and incorporating the Kolmogoroff theoryrearranging and incorporating the Kolmogoroff theory rearranging and incorporating the Kolmogoroff theoryrearranging and incorporating the Kolmogoroff theory

4/7

4/34/35

)'(882.010 L

kAf

4/710 )'(ukA

where Lwhere Lff : a characteristic length scale of the vessel (ft): a characteristic length scale of the vessel (ft)u’ : fluctuating turbulent velocity ft/secu’ : fluctuating turbulent velocity ft/secu : fluctuating turbulent velocity, ft/secu : fluctuating turbulent velocity, ft/sec : kinematic viscosity, ft: kinematic viscosity, ft22 /sec/sec

Rearranging and incorporate a correlation for turbulent fluctuation velocity Rearranging and incorporate a correlation for turbulent fluctuation velocity g g p f f yg g p f f yin a CSTR in a CSTR for feed entering at the impeller (u’’=0.45for feed entering at the impeller (u’’=0.45 DN)DN)

7/4

¶¶ Th t bli h th i llTh t bli h th i llDN

Lu f

45.0

10882.0

'5

4/34/3

¶¶ Thus, we can establish the impellerThus, we can establish the impelleragitation speed(N) to ensure thatagitation speed(N) to ensure thatmixing will not adversely affectmixing will not adversely affectthe yield, given parametersthe yield, given parameters

kA 10

the yield, given parametersthe yield, given parameters

Example 9 3Example 9 3 1 9 31 9 3 22Example 9.3Example 9.3--1, 9.31, 9.3--2 2

Fig. 9.3-8C l ti f ti i ld ffi i ith th i i t fCorrelation of reaction yield efficiency with the mixing parameters for irreversible consecutive-competitive 2nd order reaction

Effect Effect of Reactant Concentrationof Reactant Concentration( selectivity is sensitive to initial concentration since the rate of( selectivity is sensitive to initial concentration since the rate of( selectivity is sensitive to initial concentration, since the rate of( selectivity is sensitive to initial concentration, since the rate ofproduct and byproduct formation depends on concentration) product and byproduct formation depends on concentration)

For parallel irreversible reaction, the rates of formations areFor parallel irreversible reaction, the rates of formations are-- rate of product formation = rate of product formation = kkpp [R] [R] npnp

-- rate of waste generation = rate of waste generation = kkww [R] [R] nwnw

The ratio of these rates is an indicator of the selectivity toward The ratio of these rates is an indicator of the selectivity toward product formationproduct formation

][ pn kRk )(][][][ wnpn

w

p

wnw

pp R

kk

RkRk

If nIf npp > n> nww : increasing reactant concentration will increase the selectivity : increasing reactant concentration will increase the selectivity toward the product and away from the waste byproducttoward the product and away from the waste byproduct

If nIf npp < n< nww : increasing reactant concentration will decrease the selectivity : increasing reactant concentration will decrease the selectivity ff pp ww g yg ytoward the desired producttoward the desired product

Summary Summary of other methodsof other methodsTh difi ti f i i i t lTh difi ti f i i i t l There are numerous modifications for improving environmentalThere are numerous modifications for improving environmentaland economic performance of reactors and economic performance of reactors (Table 9.3(Table 9.3--2) 2)

Table 9.3-2Additional reactor operation modifications leading to pollution prevention

Table 9.3-2Additional reactor operation modifications leading to pollution prevention

9.4 Pollution Prevention for Separation Devices

Separation technologies are some of the most common and mostimportant unit operations found in chemical processes Becauseimportant unit operations found in chemical processes. Becausefeedstocks are often complex mixtures and chemical reactions arenot 100% efficient, there is always a need to separate chemical, y pcomponents from one another prior to subsequent processing steps.Separation unit operations generate waste due to their inefficiency,ddi i l i d l i h ffadditional energy input, or waste treatment to deal with off-spec

products.

In this section, we discuss the use of separation step is presented.Next, design heuristics regarding the use of separation technologiesg g g p gin chemical processes are covered. Finally, we present examples ofthe use of separation technologies for recovery of valuable

f l di ll h i icomponents from waste streams, leading eventually to their reuse inthe process.

9.4.1 Choice of Mass Separating Agent (MSA)

The correct choice of MSA to employ in a separation technology is animportant issue for pollution prevention. A poor choice may result in

i b f l f ili k b lexposure to toxic substances for not only facility workers but also consumerswho use the end product. This is especially important in food products, whereexposure to residual agents is by direct ingestion into the body with the food.p g y g y

Supercritical Fluid Extraction (SFE)D ff i t d ff b d i t t ff~ Decaffeinated coffee bean and instant coffees

~ Edible oils by volatile solvents

A poor choice of MSA may lead to excessive energy consumption and the associated health impacts of the emitted criteria pollutants (CO, CO2, NOx, SOx particular matter)SOx, particular matter).

9.4.1 Choice of Mass Separating Agent (MSA)9.4.1 Choice of Mass Separating Agent (MSA)

Choice of a mass separating agent (MSA) in an adsorptionapplication can be illustrated using a simple example.

Granular activated carbon (GAC) is a very common type ofd b t b t f th f t l it h b f d th tadsorbent, but for the recovery of metals, it has been found that

typical strong cation exchange resins have approximately a 20-fold higher capacity to adsorb Cu2+ than GAC The metal mustfold higher capacity to adsorb Cu than GAC. The metal mustbe recovered from the regenerated adsorbent using a strongacid. In this case, the use of GAC would require more energyacid. In this case, the use of GAC would require more energyconsumption and would generate more acid waste than thecation exchange resin.g

9.4.2 Process Design and Operation Heuristicsfor Separation Technologies

★ A typical chemical process can be depicted as shown in Fig. 9.4-1where reactor converts feeds to products and byproducts that must beseparated from each other by the additional input of energy.

★ Whil it b i ibl diffi lt t li i t ll t★ While it may be impossible or difficult to eliminate all wastestreams, there is every reason to be believe that wastes can beminimizedminimized

~ by judicious choice of MSA~ by correct choice and sequencing of separation technologies~ by careful control of system parameters during operation

Heuristics : 학생으로 하여금 스스로 발견케 하는 학습법

Fig. 9.4Fig. 9.4--11 Typical chemical processTypical chemical processFig. 9.4Fig. 9.4 11 Typical chemical processTypical chemical process

9.4.2 Process Design and Operation Heuristics for Separation Technologies

pp276

for Separation Technologies

★ Sequencing steps to minimize wastes in separation processes★ Sequencing steps to minimize wastes in separation processes

1 Choose the correct technology1. Choose the correct technology ~ based on the physical and chemical properties of molecules

to be separated)~ generate less waste and use less energy per unit of products

(Table 9.4-1)

2. Consider several pollution prevention heuristics to guide the design of the flowsheet and operation of the unitsg p(Table 9.4-2 : Separation heuristics to prevention pollution)

Table 9.4-1 Unit operations for separations and property differencesUnit operations for separations and property differences

Table 9.4-2Separation heuristics to prevent pollution (Muholland and Dyer, 1999)

Heuristics : 학생으로 하여금 스스로 발견케 하는 학습법

9.4.2 Process Design and Operation Heuristics f S i ifor Separation Technologies

★ Pollution prevention example for distillation

Distillation accounts for over 90% of the separation

application in chemical processing in the US. pp p g

Distillation columns contribute to process waste in 4 major waysp j y~ by allowing impurities to remain in a product~ by forming waste within the column itselfy f g f~ by inadequate condensing of overhead product ~ by excessive energy use by excessive energy use

3. Process waste by distillation

▶ Product impurities above allowable levels must be removed, l di ddi i l d ileading to additional waste streams and energy consumption

▶ Waste is formed in the reboiler where excessive temperatures and unstable materials combine to form high molecular weight tars or polymers on the heat transfer surface

▶ Condenser vent must be open to atmosphere to relieve non-condensable gases that build up in the column. If condenser duty is insufficient for internal vapor load, excess vapor will exit thevent as a waste stream

▶ Energy use leads to the direct release of criteria pollutants (CO, NOx, SOx, particular matter, VOC ) and global warming gases (primarily CO2 )

3. Example for distillation case

▶ To increase purity product, reflux ratio must increase~ this increase pressure drop across the column, raise the reboiler temperature,

d i b il dand increase reboiler duty.~ for stable materials, it may be the easiest way to decrease waste generation~ if column is operating close to flooding, reflux ratio is not an optionp g g, p

▶ Replacing existing column internals (trays or packings) with high-efficiencypacking results in greater separation for an existing column, and results in bothp g g p g ,lower pressure drop and reboiler temperature.

▶ Changing the feed location to the optimum may increase product purity.g g p y p p yreduce the loss of product to waste (30→1 lb/hr, 20% up capacity,

10% down cooling load, $9,000,000/yr benefit; Mulholland and Dyer, 1999)

▶ Insulate column, improve feed, reflux, liquid distribution; preheat column feed

▶ Withdraw product from side stream if overheads product contain light impurity▶ Withdraw product from side stream if overheads product contain light impurity(Example 9.4-1)

Example 9.4-1 Energy saving in ethanol-water distillation: side stream casedistillation: side stream case

DistillateDDxD=0.8 mole fraction

FeedF=100 moles/hr

Side StreamS=20 moles/hr

VF 100 moles/hrxF=0.4 mole fraction(½ vapor, ½ liquid)

xS=0.65 mole fraction

( p , q )V

BottomsB

0 02 mole fraction

Column design

No side stream

Side stream

L/D

2 0

2.5

D

48 72

32.56

V

96 16

63.96

QR(cal/hr)

96 16x104

63.96x104

xB=0.02 mole fractionNo side stream 2.0 48.72 96.16 96.16x10

Using a side stream design, the energy savings are 33.5%Using a side stream design, the energy savings are 33.5%

9 4 3 Pollution Prevention Examples for Separations9.4.3 Pollution Prevention Examples for Separations Pollution can be prevented using separation processes by selective

recovery and reuse of valuable components from waste streamsrecovery and reuse of valuable components from waste streams Many successful applications of separation technologies for

pollution prevention (Table 9.4-3)p p ( )

9 4 4 S t ith R t f P ll ti P ti9.4.4 Separators with Reactors for Pollution Prevention

Separators can be combined with reactors to reduce byproduct generation from reactors and increase reactant conversion to products

T bl 9 4 3 P ll ti ti l f tiTable 9.4-3 Pollution prevention examples for separation processes



Table 9.4-3 Pollution prevention examples for separation processes

Table 9.4-3 Pollution prevention examples for separation processes

9.5 Pollution Prevention Applications for Separative Reactors

An exciting new reactor type that has high potential for

p

An exciting new reactor type that has high potential for reducing waste generation

These hybrid systems combine reaction and separation in a single unit (requirements for downstream processing units aresingle unit (requirements for downstream processing units are reduced and leading to lower costs)

Key feature is the ability to control the addition of reactants and removal of product more precisely than traditional designs (Example : Rxn & adsorption of oxidative coupling of CH4 ,OCM)OCM: Oxidative coupling of methane


OCM (Oxidative coupling of methane), T=1000K

p

( p g )2CH4 +1/2 O2 C2H6 + H2O (Desired)2CH4 + O2 C2H4 + 2H2O (Desired)CH4 + 2O2 CO2 + 2H2O (Undesired)

In traditional process (Fixed bed): CH4/O2 = 50 or morehigh selectivity of C2 = 80-90%, low yield of C2 <20%

Separative reactor: Fi ed bed catal tic reactor + cooler adsorption bed= Fixed bed catalytic reactor + cooler adsorption bed

yield of C2 =50-65%

Fig. 9.5-1 SCMCR for oxidative coupling of methane

9.5 Pollution Prevention Applications for Separative Reactorsp

Fig. 9.5-1 SCMCR for oxidative coupling of methane


Membrane Separative Reactor

p

p

Thermodynamically-limited reactants (C6H12 C6H6+3H2)y y ( 6 12 6 6 2)

Parallel reactions in which the product formation has a lower order than byproduct generation

Series reactions such as selective dehydrogenations and partial oxidations (Ex: ethane oxidative dehydrogenation to th l d h d ti f th lb t d tethylene; dehydrogenation of ethylbenzene to produce styrene,

conversion 70% compare to traditional 55%)

Series-parallel reaction

9.6 Pollution Prevention in Storage Tanks and Fugitive Sources

9.6.1 Storage Tank Pollution Prevention

Storage Tanks and Fugitive Sources

Continual occurrence of air emissions of VOC from roof vents and periodic removal of oil sludge from tank bottoms

Tank bottoms are solids or sludges composed of rusts, soil particles, heavy feedstocks, other dense materials.various methods dealing with these wastesvarious methods dealing with these wastes

- periodically removed and treated via land application or disposed as hazardous waste

ti di t ti b ti f i- preventing sedimentation by action of mixers- emulsifying agents

Air emissions of VOC are major airborne pollution (standing lossj p ( gand working losses)

- Emission depends on vapor pressure, geographical location.- types of tanks loss mechanisms and reduction measures- types of tanks, loss mechanisms and reduction measures

(Table 9.6-1) (Example 9.6-1)

Table 9.6-1 Storage tank types and pollution reduction strategies

Table 9.6-1 Storage tank types and pollution reduction strategies

External Floating Roof (EFR)An external floating roof tank is a storage tank commonly used to store large quantities ofvolatile petroleum products such as crude oil or gasoline (petrol). It comprises an open-topped cylindrical steel shell equipped with a roof that floats on the surface of the storedliquid. The roof rises and falls with the liquid level in the tank.

Internal Floating Roof (IFR)An internal floating roof tank has both a permanent fixed roof and a floating desk inside. Theterm "deck" or "floating roof" is used in reference to the structure floating on the liquid storedwithin the tank. The deck of an internal floating roof tank rises and falls with the liquid levelwhilst in full contact on the underside thus achieving no vapor zone.

9.6.2 Reducing Emissions from Fugitive Sources

F i i i i i l d l Fugitive emission sources includes valves, pumps,piping connectors, relief valves, sampling connections,compressor seals, open-end lines

Leaks near seals, valve packings and gaskets aremajor issuesmajor issues

Exact timing and location and rate of release is Exact timing and location and rate of release isdifficult to predict (low-level leaks and episodic

dd l k )sudden leaks)

Fugitive Emission Profiles

Average rate of emission of VOC from fugitive components of different types can be vary significantly within a given facility(Demonstration) Estimate emission rate from 2 units at a refinery: a cracking unit and a hydrogen plant (Table 8.3-3)g y g p ( )

Equation to calculate emission rate for each component q p

E = mVOC favVOC av

where E : the emission rate(kg/hr/source), ( g ),mVOC : the mass fraction of VOC

fav : the average emission factor

Fugitive sources and contribution to emission from the 2 processing units are in Table 9.6-2

Table 9.6-2 Distribution of fugitive components and emission rates from a cracking unit and a hydrogen plant at a refineryfrom a cracking unit and a hydrogen plant at a refinery

Valves

Methods to Reduce Fugitive Emissions

two methods for reducing or preventing emissions and leaks from fugitive sources leak detection & repair (LDAR) of leaking equipment leak detection & repair (LDAR) of leaking equipment equipment modification or replacement

with emissionless technologiesg1. LDAR Program to reduce Fugitive Emissions equipments (pump, valve) are monitored periodically using an

i l (OVA i di i dorganic vapor analyzer (OVA : it arrow directing to suspected source of leakage, i.e., packing nut on a valve, at a shaft seal on a pump a gasket or weld on a flange or connector etc )pump, a gasket, or weld on a flange or connector, etc.)

EPA Guidance documents (if the source registers an OVA reading over a threshold value (>10,000), the equipment is said to be ( ) q pleaking and repair is required )

OVA screening values and EPA emission correlations (Table 9.6-3)I d t i l OVA tl i th i f f Industrial OVA programs vary greatly in their frequency of monitoring and in their effectiveness

Table 9.6-3C l ti f ti ti f iti i i d th i d f lt lCorrelations for estimating fugitive emissions and their default values

2. Equipment Modification to Reduce Fugitive Emissions I l d i i th t it h f i f Involve redesigning a process so that it has fewer pieces of

equipments and connections ( and replacing leaking equipment) Equipment replacement (valves, connectors, flanges, compressors qu p e t ep ace e t (valves, connecto s, flanges, comp esso s

and pumps) Connectors are the most numerous pieces of equipment (threaded p q p (

pipes, couplers, nut-and-ferrule types, etc.) Seals around moving parts are common locations of leaks for

f i ifugitive sources Emissions control effectiveness of various emission reduction

measure (Table 9.6-4)( ) Leakless technologies are 100 % effective in eliminating emissions,

but are very expensive to purchase and maintain (10 times than LDAR cost)LDAR cost)

Fugitive emission in petroleum refinery can be reduced ~ 70 % by using most effective reduction techniques~ 60-70 % for SOCMI (Synthetic Organic Chemical Manufacturing

Industry) facilities

Table 9.6-4Effectiveness of various fugitive emission reduction techniquesEffectiveness of various fugitive emission reduction techniques

9.7 Pollution Prevention Assessment Integrated with HAZ-OP Analysis

(The hazard and operability study is a formal procedure to identify potential hazards)

y

HAZ-OP is a procedure (identify potential hazards, investigate causeand define the consequences)

The study is apply to existing process units to improve safetyf Th HAZ OP h d i l i f id dperformance. The HAZ-OP method is to apply a series of guide words

to the process design intention

Example of process intentionli t fl th h t di till ti d- cooling water flow through a reactor or distillation condensers

- inerting system for a reactor, separator, or storage tankair supply for pneumatically driven process control valves- air supply for pneumatically-driven process control valves

9.7 Pollution Prevention Assessment Integrated with HAZ-OP Analysiswith HAZ-OP Analysis

Guide words associated with the process intentions (Table 9.7-1)

guideword, “NO” in the process intention, “cooling water flowthrough a reactor” indicate a loss of cooling waterthrough a reactor indicate a loss of cooling water.

Consequence : overheated reactor run-away reaction of explosionConsequence : overheated reactor, run away reaction of explosion

It can be applied to unit to whole process pp p(Example 9.7-1) also check, (Table 9.7-2)

HAZ-OP Analysis in Storage tank

9.8 Integrating Risk Assessment with Process DesignA C St d

Th s far in Chapter 9 e ha e incorporated en ironmental health

A Case Study

Thus far in Chapter 9, we have incorporated environmental, health,and safety concerns into the design and unit operations. We haveused quantitative assessment measures only to a limited extent. It isq yvery useful to provide a more quantitative risk assessmentcapability for the evaluation and optimization of unit operations. Tothis end a screening-level risk assessment method presented inthis end, a screening-level risk assessment method presented inChapter 8.

O i li i ld b i f b dOne important application could be screening of byproductsgenerated in a chemical reactor. Decision regarding optimumreactor operation can then be made based on the risks posed by theeacto ope at o ca t e be ade based o t e s s posed by t eindividual byproducts generated rather than on just the mass rate ofgeneration for each component. The case study deals with choosingresidence time in a fluidized bed reactor for the production ofresidence time in a fluidized bed reactor for the production ofacrylonitrile

9.8 Integrating Risk Assessment with Process DesignA C St dA Case Study

Case study: Screening byproducts generated in a chemical reactor :

case study deals with choosing residence time in a fluidized bedreactor for the production of acrylonitrile; Acrylonitrile Reactor Risk-b d I O A l i f Fl idi d B d R i h C lbased Input-Output Analysis of a Fluidized Bed Reactor with Catalyst(Bi-Mo-O)

- Ammonoxidation for main acrylonitrile reaction

CH2=CH-CH3 + NH3 + 3/2O2 CH2=CH-CN +3H2O

9.8 Integrating Risk Assessment with Process DesignA Case Study

A id i f i l i il i

A Case Study

- Ammonoxidation for main acrylonitrile reaction

CH2=CH-CH3 + NH3 + 3/2O2 CH2=CH-CN +3H2OCH2 CH-CH3 + NH3 + 3/2O2 CH2 CH-CN +3H2Oacrylonitrilepropylene

- possible side reactions

CH CH CH O CH CH CHO H OCH2=CH-CH3 + O2 CH2=CH-CHO + H2O

CH2=CH-CH3 + NH3 + 9/4O2 CH3-CN + 1/2CO2 + 1/2CO + 3H2Oacrolein

acetonitrileCH2=CH-CHO + NH3 + 1/2O2 CH2=CH-CN + 2H2O

CH2=CH-CN + NH3 + 2O2 CO2 + HCN + 3H2O

CH2=CN + 3/2O2 CO2 + CO + HCN + H2O

Case Study : Acrylonitrile

Assumed reaction model : 1st order kinetics for reactant, product and byproduct species

Model of Fluidized Bed Reactor was used to predict the effects of reaction temperature, and residence time on the generation of reaction byproducts in acrylonitrile reaction.yp y

FBR model predicts the optimum residence time for minimum waste generation and acceptable economic performance. The evaluation is based on both mass generation as well as risk generation approaches.based on both mass generation as well as risk generation approaches.

Predicted Results for FBR Model1 Effect of residence time on concentration of propylene ammonia1. Effect of residence time on concentration of propylene, ammonia,

product, and byproduct (Fig. 9.8-1) 2. Effect of residence time on the conversion of propylene to products

(Fig 9 8 2)(Fig. 9.8-2)3. Total environmental index as a function of residence time to AN

production via the ammonoxidation pathway (Fig. 9.8-3)4. Raw material costs per mass of AN produced in ammonoxidation

of propylene (Fig. 9.8-4)

Based on these results, Hopper recommended that operation reactor at temperature of 400-480oC, with

residence time of 2-10 seconds.(1992)

Fig 9.8-1Effect of reactor residence time on the conversion of propylene and ammonia to

d ( ) d b d h d l i l idi d b dproduct(AN) and byproducts . The model is of a fluidized-bed reactor at 400 C. Byproduct generation is shown on a mass basis

Using Fig 9 8 1Using Fig. 9.8-1,Equation 8-2, and

Table 8.2-11

Fig 9.8-2Eff t f t id ti thEffect of reactor residence time on the conversion of propylene to product(AN) and byproducts . The model is of a fluidized-bed reactor at 400 Cfluidized bed reactor at 400 C. Byproduct generation is shown on risk basis

Residence time of 10 secondsResidence time of 10 seconds is logical operating point for

this reactor and reaction system

Fig 9.8-3The total environmental index (eq. 8-2) as a function of reactor residence time for AN production via the ammonoxidation pathway

Fig 9.8-4gRaw materials costs per mass of acrylonitrile produced in a ammonoxiation of propylene

S O C S SSUMMARY OF CASE STUDY

1. This case study demonstrates that “tier 1” environmental assessment from Chapter 8 combined with a screening economic analysis provides valuable insights into the overalleconomic analysis provides valuable insights into the overall performance of the reactor design (residence time).

2. There is little economic benefit in operating at residence times greater than 10 seconds, and the total environmental risk index is a maximum at 10 seconds. Therefore, a residenceindex is a maximum at 10 seconds. Therefore, a residence time of 10 seconds is a logical operating point for this reactor and reaction systemand reaction system.

Questions for Discussion

1. This chapter has considered pollution preventionalternatives for a variety of types of unit operations-reactors,separation devices, storage tanks and others. For a typicalchemical process, which of these will be responsible for themajority of the emissions ? Does your answer depend onh f ll h i l di hi hthe type of pollutant or the environmental medium to whichthe pollutant is released

2. The pollution prevention alternatives identified herefrequently result in reduced energy use and reducedfrequently result in reduced energy use and reducedmaterial use. If the environmental improvements result in adesign that uses less energy and less materials why mightdesign that uses less energy and less materials, why mighta design engineer not choose to implement these options ?

Questions for DiscussionQuestions for Discussion

3. Pollution prevention alternatives have been considered for individual unit operations. Is it possible that a p ppollution prevention strategy implemented for one unit

ti i t d i i i thoperation may increase wastes and emissions in another unit operation ? (Consider, the addition of emulsifiers in tanks to reduce solids formation. Can you think of other examples ? )examples ? )

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Ch t 9Chapter 9 - Seoul National University